1. Field of the Invention
The present invention relates to a light-emitting device having a semiconductor nanocrystal layer and a method for producing the light-emitting device. More particularly, the present invention relates to a light-emitting device having a semiconductor nanocrystal layer whose voids are filled with a filling material, and a method for producing the light-emitting device.
2. Description of the Related Art
The control over the size of semiconductor nanocrystals exhibiting quantum confinement effects and enables control of electrical and optical properties of the nanocrystals. Based on this characteristic, semiconductor nanocrystals are applied to a variety of devices, including light-absorbing devices and light-emitting devices. To exhibit inherent characteristics of nanocrystals in various devices, it is necessary to uniformly arrange and form the nanocrystals into a thin film. However, since nanocrystals have a strong tendency toward aggregation between the nanocrystal particles to form aggregates due to their inherent characteristics, the nanocrystals are not formed into monolayers and empty spaces (i.e. voids) not occupied by the nanocrystals remain within nanocrystal layers.
When empty spaces are present between the nanocrystal particles of a nanocrystal layer used as a light-emitting layer in an electroluminescence device, holes do not combine with electrons at the nanocrystal particles to form excitons, and as a result, light emission is not achieved and current leakage occurs.
A detailed explanation thereof is suggested with reference to FIG. 1. FIG. 1 is a cross-sectional view schematically showing the structure of an electroluminescence device using a nanocrystal layer with voids as a light-emitting layer.
As shown in FIG. 1, the electroluminescence device 600 includes an anode 100, a hole transport layer 200, a semiconductor nanocrystal layer 300, an electron transport layer 400, and a cathode 500. When a voltage is applied to the device, the anode 100 injects holes, such as along paths 700, into the hole transport layer 200 and the cathode 500 injects electrons, such as along paths 800, into the electron transport layer 400, such that the holes meet the electrons as shown by 900, thus defining regions where excitons are recombined to emit light. The injected holes and electrons migrate toward the oppositely charged electrodes 500 and 100 and are concentrated at a semiconductor nanocrystal 310 to form excitons, after which the excitons are recombined to emit light. However, when the electrons meet the holes at voids 320 of the semiconductor nanocrystal layer 300, the electrons are directly bonded to the holes and current leakage is caused, leading to a reduction in the luminescence efficiency of the light-emitting device. In addition, since the resistance at the voids 320 is extremely low, a large amount of current is concentrated on the voids 320 by Ohm's law, thus shortening the service life of the device and causing poor electrical stability of the device.
To solve these problems, attempts have been made to reduce the volume of empty spaces formed within nanocrystal layers by increasing the coverage of nanocrystals on the nanocrystal layers. However, conventional processes have not been successful in the production of nanocrystal thin films free of empty spaces in view of their inherent characteristics.
One exemplary process for producing nanocrystal thin films is a Langmuir-Blodgett (“LB”) process wherein thin films are formed at the interface between an aqueous solution and air. However, since the Langmuir-Blodgett process utilizes weak van der Waals interactions between particles or between particles and substrates, the transfer ratio, a value representing the degree of transfer of particles to substrates, is not higher than 1. This low transfer ratio causes occurrence of a large number of defects on nanocrystal thin films and makes it impossible to produce uniform monolayers.
Another example is a dipping process wherein a substrate is repeatedly dipped in an aqueous solution of particles to increase the coverage of the particles adsorbed to the substrate. This process, however, has a problem that the coverage is limited to a maximum of 70% despite the repeated dipping.
Another example is an electrostatic self assembly process wherein particles and a substrate are oppositely charged to form a thin film. Since this process, in practice, causes the formation of nanoparticle aggregates, which leads to the occurrence of defects, it is disadvantageous in terms of degree of completeness.
In addition to the aforementioned processes, pyrolysis, laser ablation, and chemical vapor deposition (“CVD”) are known wherein nanoparticles are directly formed on a substrate through a vapor phase reaction using raw materials supplied in a gaseous state and grown by filling to arrange the nanoparticles on the substrate. However, these processes are not suitable for the production of uniform monolayers in which nanocrystals are uniformly applied.
A detailed explanation thereof is suggested with reference to FIG. 2. FIG. 2 is a transmission electron microscopy (“TEM”) image of a nanocrystal (PbSe) thin film produced by spin coating. The image shows that many irregular voids are present between PbSe nanocrystal particles on the PbSe thin film. At this time, the coverage of the nanocrystal on a substrate is about 91%. That is, the nanocrystal thin film contains a number of voids. When the nanocrystal thin film having voids is used to produce an electroluminescence device, a large amount of current is concentrated on the voids, thus shortening the service life of the device and damaging the electrical stability of the device.
Some electroluminescence devices using nanocrystals as light-emitting materials are known.
For example, U.S. Patent Publication No. 2004/0023010 teaches an electroluminescence device comprising a plurality of semiconductor nanocrystals dispersed therein, however, the presence of voids between the nanocrystals and current leakage arising from the voids is not described.
Korean Patent Laid-open No. 2005-7661 discloses a method for forming metal oxide quantum dots by mixing an insulator precursor and a nanometer-sized metal powder and annealing the mixture, and a polymer thin film containing the metal oxide quantum dots dispersed therein. Although the quantum dots and an insulator coexist in the polymer thin film, the quantum dots are not formed into a monolayer but are three-dimensionally randomly arranged with the insulator in the polymer thin film. Accordingly, in the case where the polymer thin film is used as a light-emitting layer of an electroluminescence device, the luminescence efficiency of the device is considerably low, making the polymer thin film unsuitable for the production of light-emitting devices.
Korean Patent Laid-open No. 2004-98798 discloses a high-luminance light-emitting device including an active layer made of quantum dots. This patent publication describes the presence of a dielectric layer formed on the active layer made of quantum dots. However, no mention is made in the publication regarding the presence of voids between the quantum dots and current leakage arising from the voids.
As described above, the prior art light-emitting devices containing nanocrystals have focused on increasing the coverage of the nanocrystals, however a light-emitting device having a semiconductor nanocrystal layer exhibiting high efficiency and excellent electrical stability has not been obtained.